Order of instances matters in insert_overlapping

The order you pass instances to insert_overlapping determines the result.

If you modify the TH_Overlaps test to be something like this

{-# LANGUAGE FlexibleInstances #-}
module Oops where

class C3 a where c3 :: a

instance {-# OVERLAPS #-} C3 [[a]]    where c3 = [[]]
instance                  C3 [[Int]]  where c3 = [[1]]
instance                  C3 [a]      where c3 = []

test3 :: [[Int]]
test3 = c3

the program fails with:

S1.hs:11:9: error:
    • Overlapping instances for C3 [[Int]] arising from a use of ‘c3’
      Matching instances:
        instance C3 [[Int]] -- Defined at S1.hs:7:27
        instance C3 [a] -- Defined at S1.hs:8:27
    • In the expression: c3
      In an equation for ‘test3’: test3 = c3
   |
11 | test3 = c3
   |         ^^

but if I swap the order of the instances, it compiles.

{-# LANGUAGE FlexibleInstances #-}
module Oops where

class C3 a where c3 :: a

instance                  C3 [a]      where c3 = []
instance {-# OVERLAPS #-} C3 [[a]]    where c3 = [[]]
instance                  C3 [[Int]]  where c3 = [[1]]

test3 :: [[Int]]
test3 = c3